The present invention relates generally to position control systems used in connection with rotating disk data storage systems. More particularly, the present invention relates to a method of and system for moving a memory data device member, such as a read/write head, relative to a desired concentric track of the rotating data disk storage media, and keeping the moved head in the desired alignment with the chosen concentric track.
Disk drives normally employ coarse positioning subsystems for moving heads to the proximity of addressed tracks on recording surfaces and fine positioning sub-systems for moving those heads into final alignment with the addressed tracks. Two basic categories of head positioning servo systems are known, open loop systems and closed loop systems. In an open loop head positioning system, both the coarse and fine positioning are controlled by indexing mechanisms that are mechanically or electrically coupled to the carriage assembly for the heads.
While such approaches tend to be of low cost, the disadvantage of all open loop systems is that such systems actually position the carriage assembly and heads with reference to a stationary part of the disk drive, but not with reference to recorded data on the recording surfaces. If any tendency exists for mechanical drift or movement in any part of the mechanism involved in the positioner or disk system, which can easily occur due to thermal expansion or physical abuse during shipment, installation or use of the disk drive, the indexing mechanism can position the carriage assembly exactly where it is supposed to be but the heads will remain offset from the center of the data tracks recorded prior to that mechanical movement or drift.
Even if the heads are located exactly as designed relative to the indexing mechanism, such a system requires that the data tracks be spaced far enough apart to take into account all of the variations of the system, including mechanical tolerances in the stepping motor actuator, thermal expansion of the disk, and disk runout. Thus, open loop head positioning servo systems are satisfactory only for disk drives in which the number of tracks per inch (track density) is low. That results in a disk drive product which, while effective as a low cost unit, lacks the data storage capacity of the more expensive units with the result that the cost of storage per bit stored of a low cost drive approaches the same cost as the earlier, and more expensive, dedicated servo system storage unit.
The heads of disk drives which read or write at higher track densities are generally positioned with reference to the information written on the recording surfaces of the disks, rather than with reference to a stationary support on the disk drive. Such systems are referred to as closed loop systems.
In one such implementation of a closed loop system, which utilizes a generally higher cost approach, one surface of a disk is dedicated to tracks of pre-recorded servo information. A read-only transducer or servo head is used to detect the prerecorded servo information on a selected servo track in order to provide signals which are processed by logic circuits. Since all of the other heads of the disk pack, including the servo head, are contained in a mechanically ganged head assembly, they follow the movement of the servo head as it tracks the selected servo track.
A disadvantage of that type of closed loop system is that the servo head and the data heads may not be aligned with one another due to manufacturing process tolerances or abuse of the head assembly. Thus, even though the centerline of the servo track in a selected cylinder may be followed by the servo head, the data heads may be permanently offset by differing amounts from the centerlines of the data tracks in the same cylinder.
In another type of closed loop head positioning servo system, which is known as a single surface system, the servo information is recorded on the same surface on which data is to be recorded. In such systems, the servo information is recorded in a number of servo tracks separated from one another by one or more data tracks. A single head can thus be used to read both data and servo information in a single channel, that channel consisting of a data track and two parallel servo tracks on opposite edges of the data band.
Such servo tracks generally comprise two halves, the two halves of the servo track being capable of discrimination during readback by virtue of predetermined frequency or timing relationships. The data head can thus be centered exactly on the servo track by obtaining an equal amplitude output from both halves of the track. By positioning the actuator exactly on the center of the inner and outer servo tracks in turn, that information may be used to calibrate a secondary transducer, such as a stepper motor used in the micro-step mode or an optical transducer. That approach has the disadvantage that it is necessary to move the heads away from the data band to perform the recalibration, causing an interruption in data transfer. Also, the correction factor for each track in the data band must be predicted from an algorithm which is of limited accuracy.
A variation of the single surface system described above utilizes pre-recorded servo information on each track used for data on a limited number of radial sectors on the disk surface. Data is written on the sectors of the disk surface between those servo sectors. A single transducer may thus be used to read and write data and also to detect the pre-recorded servo information as the transducer passes over a servo sector.
Such single surface systems have disadvantages, relative to the dedicated servo system described earlier, in that they are subject to catastrophic write failures. For example, if a head erroneously erases data from the recording surface, the servo information on the surface can be obliterated. That results in a partial or even total loss of servo capability. The dedicated servo system, on the other hand, is not subject to such catastrophic write failure since its servo head is a read-only transducer.
Other disadvantages of a single surface system are the reduction of the data storage capacity of the surface, since a significant portion of the data surface is normally dedicated to the recording of servo information and causes total inflexibility in the length and number of sectors recorded on the disk surface since the servo data is factory pre-recorded and data must fit between the servo bursts. On the other hand, if a sufficient number of servo bursts are written per track, then the servo system can be designed to follow eccentricities in each track, making the system suitable for very high track density applications.
Another approach used by the prior art is that shown in U.S. Pat. No. 4,396,959 to Harrison et al. The device shown therein utilizes a position transducer which provides a poly-phase signal which is generated in response to the actual sensed present position of the head support structure relative to the frame to which the head support structure is rotatably mounted. That structure serves as the coarse head positioning servo transducer.
A fine position closed loop servo is also provided which is connected to the driver of the head assembly and is operated from pre-recorded information in a single, data masked servo sector on a data surface of the rotating disk. That data is read by a head supported by the moveable member or head support structure.
As disclosed in Harrison et al, a single-disk surface on one of the many disks utilized for storage in the disk drive contains a single 200 byte wide servo data sector which utilizes two similar bursts B1 and B2. Each odd-numbered track has a burst of data Bl spaced radially away from the track and centered on an imaginary centerline between the track and the adjoining lower-numbered track. The second data burst B2 is likewise spaced about an imaginary line midway between the centerline of each even numbered track and the next lower numbered track. Each of the data bursts are displaced in time from each other.
The Harrison et al device utilizes only one head in order to provide fine correction for the entire disk stack head assembly. As the head passes over the first burst B1, it is read and its peak is detected and integrated in order to produce an amplitude signal. Shortly thereafter, the second data burst is read by the same head and is likewise peak-detected and integrated to produce a signal representative of its amplitude. Those amplitudes are then individually sampled and held separately for comparison. The resulting signal is then converted to a digital signal for use by the microprocessor servo controller.
The system disclosed by Harrison et al has the same disadvantages as the single surface systems described above, namely that it is subject to a catastrophic write failure in the event that the single head reading the only recording surface containing the servo information either erroneously erases data from that recording surface or that recording surface is otherwise damaged during shipment or by other abuse.
Another disadvantage of the Harrison et al device is that it is not sufficiently accurate and reliable enough when used for the positioning of transducers with high capacity disk drives having, for example, more than six hundred concentric tracks per inch.
With the introduction of both the mini or 51/4 inch Winchester disk drive systems and the micro or 3 and 1/2 inch Winchester disk drive systems, and the accompanying data storage which has been achieved by the instant assignee, the correct positioning of the read/write transducer over a centerline of the addressed track has become even more critial.
Among the various advances that have been made in the construction and operation of disk drive systems are those developments made by the assignee of the present invention, RODIME PLC, which developments are set forth in the following United States Patent and patent applications. The subject matter of such patent and patent applications is hereby incorporated by reference.
U.S. Pat. No. 4,392,095, entitled, "METHOD OF AND APPARATUS FOR GENERATING A UNIQUE INDEX MARK FROM THE COMMUTATION SIGNAL OF A D.C. BRUSHLESS MOTOR," discloses a system for providing a unique index mark relative to the computer disk which is required for avoiding errors in addressing a memory location on the disk surface. That index mark is provided by correlating the commutation signal from a d.c. motor with a synchronizing signal present on one or more discrete tracks of the computer disk.
U.S. patent application Ser. No. 332,003, entitled "READ/WRITE HEAD THERMAL COMPENSATION SYSTEM," now abandoned discloses a thermal compensation system used by RODIME PLC in its 5 and 1/4 inch disk drive system. That thermal compensation system uses different materials with different coefficients of thermal expansion for various components of the positioning mechanism for the read/write head used in the disk drive system.
U.S. Pat. No. 4,538,192, entitled "VENTILATION SYSTEM FOR A COMPUTER DISK DRIVE HUB ASSEMBLY," discloses a ventilation system for use in a computer disk drive which enables the disk file data storage capacity to be increased for a given volume of chamber housing the disk and improves the disk drive operating performance.
U.S. Pat. No. 4,489,259, entitled "METHOD AND APPARATUS FOR CONTROLLING A STEPPER MOTOR," discloses a system for minimizing oscillations of the stepper motor for a single step, minimizing the time taken for the stepper motor to move between tracks for multi-track seek operations and reducing the angular hysteresis due to the mechanical and magnetic properties of the stepper motor construction. In controlling the operation of the stepper motor, a microprocessor circuit is adapted to drive the stepper motor in accordance with predetermined programs.
U.S. Pat. No. 4,568,988, entitled "MICRO-HARD DISK DRIVE SYSTEM," discloses a high-density micro-Winchester hard disk system using a hard disk of approximately three and one-half inches and having digital information stored at a density of approximately 600 concentric tracks per inch. A stepper motor is designed to increment in steps of 0.9.degree. which causes the read/write head to move from one track to the next adjacent track on the hard disk. For many applications, it has now become advantageous to utilize a high performance three and one-half inch Winchester disk drive having a storage capacity in excess of that achieved in the above-disclosed micro hard disk drive system, namely in excess of 10 Megabytes. Such increased storage capacity allows the use of more sophisticated programs and the storage of data for use therewith than has been possible heretofore.